Centrifugal Fan

ABSTRACT

A centrifugal fan  1  as an example of the invention includes: an impeller  2  having a cylindrical outline shape with an outer peripheral diameter of 25 mm or less; a motor  3  rotating the impeller  2;  and a housing  4  accommodating the impeller  2  therein. The motor  3  is a three phase motor and rotates the impeller  2  around the central axis  10  thereof at 10000 or more revolutions per minute. Since the motor  3  is a three phase motor, an utilization efficiency of coils is higher, which decrease a heat generation of a stator  38.  Besides, the motor  3  is driven so that three coils are power supplied at all times, thereby further decreasing a heat generation of the stator  38.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an electric centrifugal fan, and particularly, to a centrifugal fan used for cooling an electric product and electronic equipment.

2. Description of the Related Art

In recent years, reduction in size of a cooling fan incorporated in electronic equipment has been increasingly demanded in company with down-sizing and increased performance of electronic equipment. Such a fan has been required to have a hydrostatic pressure and an air capacity both at a comparative high level.

For example, a heat sink fan for cooling MPU (Micro Processing Unit) has been proposed. The heat sink fan includes: a radiation heat sink disposed on MPU and a cross-flow fan installed on the top of the heat-sink. The heat sink is aggressively cooled by cooling-air from the cross-flow fan.

Since the heat sink fan is, however, a cross-flow fan, so much high hydrostatic pressure cannot be obtained and a high cooling effect cannot be ensured. Though the centrifugal fan can achieve a hydrostatic pressure higher than a cross-flow fan, the centrifugal fan is further required to have improvement on a shape and a further higher hydrostatic pressure in a case where the fan is incorporated in smaller-sized electronic equipment.

Another proposal has been made on a centrifugal fan having plural vanes constituting an impeller. The centrifugal fan is designed so that a radius of a circle defined by the outer periphery of plural vanes is smaller than a length of the impeller in the axial direction thereof to thereby render a sectional area in a plane perpendicular to the axial direction smaller so as to be incorporated in portable electronic equipment.

In a centrifugal fan in such an application, since a motor rotating at a high speed is accommodated in a small housing, heat generated from stator coils and a driving circuit of the motor is accumulated in the housing to raise a temperature in the housing to a great extent. Therefore, the motor is heated at a high temperature in a high speed rotation, which, in some case, causes a thermal shutdown function of the driving circuit to be activated to stop the motor. Therefore, it is very important that a heat generation of the motor is suppressed to thereby protect the driving circuit from heat.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a smaller-sized centrifugal fan capable of raising a hydrostatic pressure and increasing an air capacity.

It is another object of the invention to provide a smaller-sized centrifugal fan with a motor having a suppressed heat generation.

It is still another object of the invention to protect a driving circuit of a motor for a smaller-sized fan from heat.

A centrifugal fan as an example of the invention includes an impeller having a cylindrical outline shape, a motor connected to the impeller and rotating the impeller at 10000 or more revolutions per minute around the central axis thereof, and a housing accommodating the impeller therein.

The motor includes a rotor assembly supporting the impeller and having a rotor magnet disposed around the central axis thereof, and a stationary assembly having a stator generating a rotational torque between the rotor magnet and the stator.

The impeller includes plural vanes. The each vanes has a connection end at a lower and an upper end, and is disposed at an equal interval each other as a manner that the connection end is fixed on an upper surface of the rotor assembly. That each vane is disposed parallel to the central axis having a predetermined distance from the central axis in radial direction, and the upper end of each vane is fixed on a ring as an open end.

The outline shape of the impeller satisfies relations of 2≦h/r≦20 and 2r≦25 mm, where r is a radius of the impeller and h is a height of the impeller in axial direction.

The housing includes an intake port formed on the open end of the impeller, and an air blowing-out port formed so as to be opposite the side surface of the impeller and extending in the axial direction.

A centrifugal fan as an example of the invention cannot only raise a hydrostatic pressure and increase an air capacity, but can also decrease a heat generation of a motor. Besides the centrifugal fan cannot only protect an electronic component having a driving circuit for the motor from heat but can also be downsized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view showing a centrifugal fan related to a first embodiment of the invention.

FIG. 2 is a front view showing the centrifugal fan.

FIG. 3 is a lateral sectional view showing the centrifugal fan.

FIG. 4 is a diagram showing an electric circuit of and a driving circuit for a motor.

FIG. 5 is a waveform diagram showing a state of power supply to stator coils of the motor.

FIG. 6 is a diagram showing another electric circuit and another driving circuit for a motor.

FIG. 7 is a waveform diagram showing a state of power supply to stator coils of the motor.

FIG. 8 is a longitudinal sectional view showing a centrifugal fan of a second embodiment.

FIG. 9 is a longitudinal sectional view showing a centrifugal fan of a third embodiment.

FIG. 10 is a lateral sectional view showing a centrifugal fan of a fourth embodiment.

FIG. 11 is a longitudinal sectional view showing a centrifugal fan of a fifth embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Description will be given of an embodiment of the invention below with reference to the accompanying drawings. Note that in a case where description below is given referring to positional relationships between constituents, directions and positions, upward or downward, or to left or right, such geographical relationships are shown as positional relationships or directions on the figures, regardless of those of the constituents assembled in actual equipment.

First Embodiment

Structure of Centrifugal Fan 7

FIG. 1 is a view showing a construction of a centrifugal fan 1 related to the first embodiment of the invention and a longitudinal sectional view obtained cut along a plane including the central axis 10. FIG. 2 is a front view showing the centrifugal fan 1. FIG. 3 is a lateral sectional view showing the centrifugal fan 1.

The centrifugal fan 1 is an electric fan used for cooling, for example, electronic component in an electric product and electronic equipment (especially, of a portable types). The centrifugal fan 1 includes: an impeller 2 generating an air flow by rotation thereof; a motor 3 rotating the impeller 2; and a housing 4 not only accommodating the impeller 2 and the motor 3, but also controlling the air flow generated by rotation of the impeller 2 to blow out the air.

The impeller 2 having a cylindrical outline shape is formed with plural vanes 21 for generating an air flow; a connection section 22 not only fixedly connecting the lower end sections in the axial direction (on the lower side in FIG. 1) of the plural vanes 21, but also connected to the motor 3; and a ring 23 fixing the top ends of the plural vanes 21 in the axial direction (on the upper side in FIG. 1) and connecting the plural vanes 21 thereby. Formed integrally with the ring 23 into a single piece is a reinforcing ring 231 protruding from the outer periphery of the plural vanes 21 outwardly in the radial directions and raising a strength of the ring 23. The plural vanes 21, the connection section 22 and the ring 23 are molded in a single piece using resin.

The plural vanes 21 are, as shown in FIG. 3, arranged at a predetermined distance between adjacent vanes, apart from and leaving a clearance around the central axis 10 and, as shown in FIG. 1, extend in parallel to the central axis 10. Air flows into an internal space 90 surrounded with the plural vanes 21 from upward in the axial direction while the motor 3 rotates. That is, the reinforcing ring 231 of the impeller 2 is an open end through which air is guided into the internal space 90. The lower end of the internal space 90 in the axial direction is closed by the connection section 22 connected to the motor 3.

The motor 3 is a three phase motor rotated by three phase driving and a rotor assembly includes: a rotor york 31; a shaft 32; and a rotor magnet 35. A stationary assembly includes a base plate 36; a sleeve 34; a holder 33; a seal 37; and stator 38.

The rotor york 31 rotates around the central axis 10 (as a center) relative to the base plate 36 in the shape of approximately a plate and the base plate 36 is inserted into the housing 4, while the rotor york 31 is connected to the connection section 22 of the impeller 2. The shaft 32 is fixed to the rotor york 31 coaxially with the central axis 10 and inserted into the sleeve 34 in a freely rotatable manner. The driving field rotor magnet 35 is fixed in the inner surface of the rotor york 31. The rotor magnet 35 is constituted of a rare earth bond magnet and in this embodiment, a Nd (neodymium) bond magnet is adopted.

The base plate 36 is made of a metal (in this embodiment, stainless steel (SUS304) is adopted) and the holder 33 in which the sleeve 34 is inserted is fixed in the base plate 36. The seal 37 plugs a clearance between the shaft 32 and an opening in the top side of the holder in the axial direction. The stator 38 having stator coils for driving is fixed on the outer surface of the holder 33.

A printed circuit board 392, which is a FPC (flexible printed circuit board), is attached to the lower end surface of the base plate 36 in the axial direction. An electronic component 391 including the driving circuit for the motor 3 is mounted on the printed circuit board 392. That is, at least a mounted site of the printed circuit board 392 on which the electronic component 391 is mounted is brought into direct contact with the base plate 36. The electronic component 391 is connected to the stator 38 through the printed circuit board 392. The electronic component 391 is also in contact with (a cap 46 of) the housing 4. Note that the printed circuit board 392 may be brought into contact with the base plate 36 with a member increasing thermal conductivity such as a thermal tape, thermal grease, a adhesive or a double-sided tape interposed therebetween. The electronic component 391 and the printed circuit board 392 may be interchanged in position on the base plate 36. In this case, the electronic component 391 is brought into contact with the base plate 36 directly or with a member increasing thermal conductivity interposed therebetween.

Constituent components of the motor 3 such as the rotor york 31, the base plate 36 and the stator 38 and the electronic component 391 have all a size of an outer peripheral diameter of the impeller 2 or less. That is, the central axis of the motor 3 and the impeller 2 coincide with each other, the diameter of the motor 3 is equal to or less than the outer peripheral diameter of the impeller 2 and the motor 3 is disposed in a space of a cylindrical region having the outer peripheral diameter or less of the impeller 2.

Currents supplied to the stator coils of the stator 38 in the motor 3 are controlled by the driving circuit of the electronic component 391, and the rotor york 31 is thereby rotation-driven with the shaft 32 as a center with the help of a magnetic action between the rotor magnet 35 and the stator 38. With the rotation-driving, the impeller 2 connected to the rotor york 31 rotates around the central axis 10 as a center. A rotational direction of the rotor york 31 (that is, a rotational direction of the impeller 2) is a direction indicated by an arrow mark P in FIG. 3 and the number of revolution is 10000 or more revolutions per minute. A method for driving the motor 3 will be described later.

The rotor magnet 35 of the motor 3 is constituted of a rare earth bond magnet generating a high magnetic flux density. Hence, even a small motor 3 secures a sufficient rotation driving force at a low power, resulting in reduction in a heat generation of the stator coils. Therefore, it is prevented that the motor 3 is heated at a very high temperature to transfer heat to the electronic component 391, thereby further preventing a failure or a malfunction of the electronic component 391 due to heat from occurring. The base plate 36 is formed with a metal and the electronic component 391 is substantially brought into contact with the base plate 36. Hence, since not only heat generated in the motor 3, but also heat generated in the driving circuit of the electronic component 391 are efficiently dissipated to outside of the motor, the electronic component 391 is protected from heat to thereby prevent a failure and a malfunction thereof.

Note that the rotor magnet 35 is not made limitedly of Nd (neodymium) bond magnet, but any of other rare earth bond magnets may be used. The base plate 36 is not made limitedly of stainless steel, but any of other metals such as aluminum, aluminum alloy and copper may be used.

The housing 4 accommodating the impeller 2 and the motor 3 is in the shape of approximately a rectangular prism longer in parallel to the central axis 10 in its outline. The housing 4, as shown in FIG. 1, includes: the intake port 41 formed at a site opposite the reinforcing ring 231 (open end) of the impeller 2; and the air blowing-out port 42, as shown in FIG. 2, formed longer in parallel to the central axis 10 facing the side surface of the impeller 2. The intake port 41 is formed in the shape of a circle being approximately the same as the outer peripheral diameter of the impeller 2. The air blowing-out port 42 is, as shown in FIG. 3, expanded toward the outside of the housing 4 and reaches to the inner surface 49 surrounding the impeller 2.

The housing 4 is, as shown in FIGS. 1 and 2, constructed of a housing member 45 accommodating the impeller 2 and the main part (from the top end to a point in the vicinity of the stator 38) of the motor 3; and the cap 46 engaged with the housing member 45, and the intake port 41 and the air blowing-out port 42 are provided in the housing member 45. The housing member 45 is made of resin, for example FRP (fiberglass reinforced plastics) using PBT (polybutylene terephthalate) as the base material. The cap 46 is made of aluminum.

Since the cap 46 is made of a metal and is in contact with the electronic component 391 of the motor 3 directly (or with a member increasing the thermal conductivity interposed therebetween), heat generated in the driving circuit of the electronic component 391 is dissipated from the base plate 36 and the cap 46 to protect the electronic component 391 from heat. Note that the cap 46 is not made limitedly of aluminum, and may be made of any of metals such as copper or stainless. The housing member 45 may be made of a metal such as aluminum, copper or stainless steel instead of resin. In a case where the electronic component 391 and the printed circuit board 392 are interchanged in position on the base plate 36, the mounted site of the electronic component 391 on the printed circuit board 392 is brought into contact with the cap 46 directly or with a member increasing thermal conductivity interposed therebetween.

With a centrifugal fan 1 having the structure described above, when the impeller 2 rotates, air flows into the space 90 from the intake port 41, flows out between the plural vanes 21, is moved along the inner surface 49 of the housing 4 and is blown out through the air blowing-out port 42.

Herein, the outer peripheral diameter 2r (r is a radius) of the impeller 2 shown in FIG. 1 (and FIG. 3) is 25 mm or less and a length h of the plural vanes 21 in the axial direction is a length satisfying a relation of 2≦h/r≦20. The length is preferably a length satisfying a relation of 3≦h/r. In a case where a thickness of a recent version of a note book type personal computer is considered, the outer peripheral diameter 2r of the impeller 2 is more preferably 20 mm or less. In the first embodiment, the outer diameter 2r is 12 mm and the length h is 27 mm (wherein a width of the reinforcing ring 231 is 4 mm).

With the impeller 2 satisfying 2≦h/r adopted, a point at which the maximum flow velocity of air from blowing out between the plural vanes 21 is located in the neighborhood of the middle between both ends of each vane 21 in the axial direction. As a result, a flow rate of air is increased and a flow of air with good efficiency can be created. With the impeller 2 satisfying h/r≦20 adopted, a high speed rotation at 10000 or more revolutions per minute (for example 20000 revolutions per minute) can be realized without accompanying a vibration, a flow rate of air can be further increased by high speed rotation; thereby, enabling a hydrostatic pressure to be raised and a flow of air with good efficiency to be produced. The impeller 2 equipped with the reinforcing ring 231 suppresses deformation of the vanes 21 due to high speed rotation.

Driving Method for Motor 3 of Centrifugal Fan 7

Then, description will be given of a driving method for the motor 3 of the centrifugal fan 1. FIG. 4 is a diagram showing an electric circuit of and a driving circuit for the motor 3. FIG. 5 is a waveform diagram showing a state of power supply to stator coils of the motor 3.

The motor 3, as described above, is a three phase motor rotated by three phase driving. Three stator coils 51, 52 and 53 for generating a rotational force are, as shown in FIG. 4, connected in a Y letter (star connection) to form a three phase windings 54 and power supply to the three phase windings 54 is controlled to thereby rotation-drive the rotor assembly. Power supply to the three phase windings 54 is controlled by switching in switch sections 61, 62 and 63, each being constituted of a bipolar transistor.

The motor 3 is driven, as shown in FIG. 5, by means of a so-called 180 degree power supply scheme: that is, a terminal U of a three phase winding 54 takes potential levels such that L (low) level is kept between electric angles of 0 degree and 180 degrees and H (high) level is kept between electric angles of 180 degrees and 360 degrees and thereafter, such a change-over cycle in potential level are repeated as time goes. A terminal V leads the terminal U by 120 degrees in electric angle phase and a cycle similar to that at the terminal U is repeated with the respect to change-over of potential levels. A terminal W leads the terminal U by 240 degrees in electric angle phase and a cycle similar to that at the terminal U is repeated with the respect to change-over of potential levels. With such cycles advanced, a power supply state of the three phase windings 54 changes such that as an electric angle advances by 60 degrees, the same power supply state is repeated in a cycle of 360 degrees in electric angle.

In such a motor 3, since the terminals U and V are at L level, while the terminal W is at H level between 0 degree and 60 degrees in electric angle (over a first about ⅙ of the power supply cycle), currents flow both into the terminals U and V from the terminal W and all the coils 51, 52 and 53 are power supplied. Since the terminal U is at L level, while the terminals V and W are at H level between 60 degrees and 120 degrees in electric angle (over a second about ⅙ of the power supply cycle), currents flow into the terminal U from the terminals V and W both and all the coils 51, 52 and 53 are power supplied. Since one of the terminals U, V and W is at H level, while the other two terminals are at L level or two of the terminals U, V and W is at H level, while the other one terminal is at L level at any other values in electric angle, all the coils 51, 52 and 53 are power supplied in a similar way. In such way, in the motor 3, all the coils 51, 52 and 53 are power supplied at all times (all over the power supply cycle) if a influence of switching or the like is neglected.

The above description has given of the structure of a centrifugal fan 1, wherein an outer peripheral diameter 2r of the impeller 2 of the centrifugal fan 1 is 25 mm or less, a length h of the plural vanes 21 in the axial direction satisfies a relation of 2≦h/r≦20 and the number of revolutions of the impeller 2 is 10000 or more revolutions per minute. With such a structure adopted, reduction in size of the impeller 2 is realized and not only is a hydrostatic pressure is raised, but an air capacity is also increased. The motor 3 is of a size equal to or less than an outer diameter 2r of the impeller 2, placed in a small cylindrical region coaxial with the central axis 10 and thereby, further reduction in size can be realized.

The driving circuit for the motor is, as described above, required to be protected from heat in order to prevent a failure or a malfunction due to rise in temperature. Especially, in a case where a motor rotating at a high speed of 10000 or more revolutions per minute is accommodated in a narrow region of 25 mm or less in the outer peripheral diameter, heat generated from the driving circuit and the stator coils of the motor is easily accumulated in the housing and rise in temperature is great; therefore, it is extremely important to protect the driving circuit from heat.

Since in a centrifugal fan 1, the motor 3 is a three phase motor, a utilization efficiency of the stator coils are higher than a two phase motor used in more cases in cooling of electronic equipment, and in a case where the motor is rotated at the same number of revolutions, a heat generation of the stator 38 is decreased. As a result, the electronic component 391 is actually protected from heat of the motor 3 while sustaining a high hydrostatic pressure and a large air capacity. By protecting the electronic component 391 from heat, no necessity arises for a highly expensive driving circuit with special specifications endurable against high temperature; thereby enabling reduction in manufacturing cost of a centrifugal fan 1 to be realized.

If three phase windings in a way of which three stator coils are connected is driven by means of a 120 degree power supply scheme (see FIG. 7 described later), there arises a stator coil in a non-power supply state in continuation over an about ⅙ of a power supply cycle, (wherein if voltage waveforms at the terminals of the respective three phase windings are smoother than that of a rectangular wave, over a ⅙ or less), which reduces a driving efficiency. In a centrifugal fan 1, however, three stator coils 51, 52 and 53 are connected and driven by means of the 180 degree power supply scheme; therefore, all the coils of the motor 3 are power-supplied at all times. Hence, a current density is reduced as compared with a driving scheme in which there exists a coil in a non-power supply state over a section of a power supply cycle as in the 120 degree power supply scheme, which reduce a heat generation of the stator 38 more. Thereby, the electronic component 391 is further protected from heat.

FIG. 6 is an electric circuit diagram showing another connecting structure for the driving motor 3. FIG. 7 is a waveform diagram showing a state of power supply to stator coils of the motor of FIG. 6.

In FIG. 6, three coils 51, 52 and 53 for generating a rotational force are connected in the shape Δ (delta connection) to form three phase windings 55 and, as shown in FIG. 7, driven by means of the so-called 120 degree power supply scheme. That is, a terminal W of a three phase winding 55 takes potential levels such that L (low) level is kept between electric angles of 0 degree and 120 degrees, the winding is turned off between electric angles of 120 degrees and 180 degrees, L (low) level is kept between electric angles of 180 degrees to 300 degrees and the winding is turned off between electric angles of 300 degrees and 360 degrees, and thereafter, such a change-over cycle in potential level are repeated as time goes. A terminal V leads the terminal W by 120 degrees in electric angle phase and the terminal U leads the terminal W by 240 degrees in electric angle phase. With such cycles advanced, a power supply state of the three phase windings 55 changes such that as an electric angle advances by 60 degrees, similar power supply state is repeated in a cycle of 360 degrees in electric angle.

In such a motor 3, since the terminals U is at H level, while the terminal V is turned off and the terminal W is at L level between 0 degree and 60 degrees in electric angle (over a first about ⅙ of the power supply cycle), not only does a current flow directly to the terminal W from the terminal U, but a current also flows to the terminal W from the terminal U by way of the terminal V and all the coils 51, 52 and 53 are power-supplied. Since one of the terminals U, V and W is at H level, another thereof is turned off and the rest is at L level at any other values in electric angle, all the coils 51, 52 and 53 are power-supplied in a similar way. As a result, all the coils 51, 52 and 53 are power-supplied at all times (all over the power supply cycle) if a influence of switching or the like is neglected.

If three phase windings in delta connection is driven by means of the 180 degree power supply scheme, there arises a coil in a non-power supply state in continuation over an about ⅙ angle of a power supply cycle, (wherein if voltage waveforms at the terminals of the respective three phase windings are smoother than that of a rectangular wave, over a ⅙ angle or less). In a centrifugal fan 1, however, three stator coils 51, 52 and 53 are driven by means of the 120 degree power supply scheme; therefore, all the stator coils of the motor 3 are power-supplied at all times, being resulted in driving with a good efficiency. As a result, a heat generation of the stator 38 is reduced and the electronic component 391 is protected from heat. In the case of the motor 3 shown in FIG. 6 as well, a heat generation is reduced by using a three phase motor in a similar way to that in the case of FIG. 4.

FIGS. 8 and 10 are longitudinal sectional views showing improved structures of a centrifugal fan 1 in a case where the electronic component is necessary to be cooled more. Since the centrifugal fans of the other embodiments are equivalent to that of the first embodiment in basic structure, description will be given of different points only.

Second Embodiment

A centrifugal fan 1 shown in FIG. 8 is additionally provided with two radiation fins 71 and 72, as seen by comparison with the structure shown in FIG. 1, and the cap 46 is removed from the housing 4 and parts of the surface of the lower end in the axial direction and a side surface thereof have openings. The housing 4 is made of resin and formed with FRP including PBT as a base material.

The radiation fin 71 is attached to the electronic component 391 and exposed in the opening at the end surface of the housing 4 in the axial direction. The radiation fin 72 is attached to part of the metal base plate 36 exposed through an opening of the side surface of the housing 4. The radiation fins 71 and 72 are made of a metal (for example, aluminum).

In the centrifugal fan 1 shown in FIG. 8, since heat generated in the driving circuit in the electronic component 391 is efficiently dissipated from the radiation fin 71, the electronic component 391 has no chance to be heated at a high temperature state, preventing a failure and a malfunction to be caused by heat from occurring. Besides, since heat generated in the driving circuit and the motor 3 is efficiently dissipated from the radiation fin 72 through the base plate 36, the electronic component 391 is protected from heat with more of certainty. Note that both of the radiation fins 71 and 72 are not necessary to be provided and only one of them will do.

Third Embodiment

In a centrifugal fan 1 shown in FIG. 9, at least part of the housing 4 is made of a metal high in thermal conductivity and an opening is formed at the lower end surface in the axial direction. The electronic component 391 is attached to a metal part 401 of the housing 4 in contact therewith, directly or with a member increasing the thermal conductivity described above interposed therebetween. The printed circuit board 392 made of FPC extends along the opening at the lower end surface of the base plate 36, then bends around the opening end facing downward in the axial direction and further extends along the outer side surface of the housing 4. With such a structure, the electronic component 391 is connected to the stators 38 through the printed circuit board 392. The other parts of the construction are similar to those of FIGS. 1 and 3.

In the centrifugal fan 1 of FIG. 9, a flow of air generated in the housing 4 by rotation of the impeller 2 (that is, rotation of the motor 3) cause heat to be transferred from the electronic component 391 to the housing 4 is efficiently dissipated. By making the housing 4 of a metal, a cooling performance is further improved. With such a structure adopted, the electronic component 391 in the centrifugal fan 1 is protected from heat with certainty. A failure and a malfunction that would be otherwise encountered is prevented from occurring. Note that, though being not shown in FIG. 9, a protective cover covering to protect the electronic component 391 may be provided. The electronic component 391 may be mounted on the surface opposite the housing 4 side (the upper surface of the printed circuit board 392 of FIG. 9). In this case, a site on the printed circuit board 392 where the electronic component 391 is mounted is brought into contact with the metal part 401 of the housing 4, directly or with a member increasing the thermal conductivity interposed therebetween.

Fourth Embodiment

In a centrifugal fan 1 shown in FIG. 10, an auxiliary space 81 surrounded with a cover 80 is provided outside of the housing 4 and the electronic component 391 is attached to the inner wall surface of the auxiliary space 81. The auxiliary space 81 is provided on the outside of a region serving as a flow path of air in the housing 4 and the housing 4 is further provided with plural holes 82 communicating between the auxiliary space 81 and the flow path of air in the housing 4.

In the centrifugal fan 1 shown in FIG. 10, a part of a flow of air generated in the housing 4 by rotation of the impeller 2 (that is, rotation of the motor 3) flows into the auxiliary space 81 through some holes 82 and out from the auxiliary space 81 through other holes 82. Thereby, a flow of air is generated in the auxiliary space 81 and, efficiently, heat generated from the electronic component 391 is removed by the air flow. Note that the cover 80 is made of a metal, which promotes cooling of the electronic component 391 more.

Fifth Embodiment

FIG. 11 is a centrifugal fan showing the fifth embodiment. Since the centrifugal fan 1 of the fifth embodiment is equivalent to the first embodiment in basic construction, description will be given of different parts.

In the centrifugal fan 1 shown in FIG. 11, a dimension of a rotor magnet 135 in the axial direction is designed to be approximately the same as that of a stator core 40 constituting stators 138. With such a construction adopted, since the dimension of the rotor magnet 135 in the axial direction can be shorter than the rotor magnet 35 of FIG. 1, the rotor assembly can be more or less lighter in weight.

While description has been given of the centrifugal fans 1 related to the embodiments of the invention, the invention is not specifically limited to the embodiments and various modifications or alterations thereof can be implemented.

For example, the number of stator coils of a motor 3 is not limited to 3 and a three phase motors in number of a multiple of 3 equal to or more than 6 may be employed. Even with such a three phase motor adopted, a heat generation of the stator 38 is reduced as compared with that of a two phase motor and by driving all the stator coils of a motor so as to be power-supplied at all times, a heat generation of the stator 38 is further reduced. A structure of the interior of a motor 3 can be altered in a proper way and, for example, the sleeve 34 and the stators 38 may also be arranged so as to be parallel to each other. Switching elements other than bipolar transistors may be employed as the switch sections 61, 62 and 63.

While in FIG. 1, the cap 46 of the housing 4 is formed with a metal and in FIG. 9, description is given that the housing is preferably formed with a metal, an effective cooling of the electronic component 391 can be realized by forming at least a part of the housing 4 with a metal and bringing the electronic component 391 into contact a metal site of the housing.

A sectional shape of a vane 21 of the impeller 2 is not limited to the shape exemplified in FIG. 3 and the shape may be flat. The vanes 21 may be made not of resin, but of a metal. An outline shape in a section perpendicular to the central axis 10 of the housing 4 is not necessary to be a rectangle exemplified in FIG. 3 and an unnecessary corner may be rounded in a proper way. Sectional shapes of the air blowing-out port 42 and the inner surface 49 are not limited to those exemplified in FIG. 3 and may be deformed in a suitable way in consideration of a air blowing-out efficiency. The reinforcing ring 231 is not limited to the shape of a cylinder and may take a thick annular shape, and the top end of a vane 21 in the axial direction is not of a structure in which the reinforcing ring 231 is as shown in FIG. 1 attached on the inner side, and may also be connected to the lower end surface of the reinforcing ring 231. 

1. A centrifugal fan comprising: an impeller having a cylindrical outline shape; a motor connected to the impeller and rotating the impeller at 10000 or more revolutions per minute around a central axis thereof; and a housing accommodating the impeller therein, wherein the motor includes: a rotor assembly supporting the impeller and having a rotor magnet disposed around the central axis thereof; and a stationary assembly having a stator generating a rotational torque between the rotor magnet and the stator, wherein the impeller includes plural vanes, each vane has a connection end at a lower and an upper end, and is disposed at an equal interval each other as a manner that the connection end is fixed on an upper surface of the rotor assembly, that each vane is disposed parallel to the central axis having a predetermined distance from the central axis in radial direction, and that the upper end of each vane is fixed on a ring as an open end, and the outline shape of the impeller satisfies relations of 2≦h/r≦20 and 2r≦25 mm, where r is a radius of the impeller and h is a height of the impeller in axial direction, and wherein the housing includes: an intake port formed on the open end of the impeller, from which an air is absorbed into the impeller when the impeller rotates; and an air blowing-out port formed on the housing, from which the air is flowing out to an outside of the centrifugal fan when the impeller rotates.
 2. The centrifugal fan according to claim 1, wherein electric currents are supplied at all times in all stator coils wound around the stator as a three phase motor.
 3. The centrifugal fan according to claim 1, wherein the rotor magnet is a rare earth bond magnet.
 4. The centrifugal fan according to claim 2, wherein the stationary assembly further includes: a printed circuit board attached on the stationary assembly; an electronic component having a driving circuit for the three phase motor and mounted on the printed circuit board; and a metal base plate with which the electronic component is in contact.
 5. The centrifugal fan according to claim 4, wherein the stationary assembly is provided with a radiation fin attached to the base plate for radiating heat.
 6. The centrifugal fan according to claim 2, wherein the stationary assembly further includes: a printed circuit board attached on the stationary assembly; an electronic component having a driving circuit for the three phase motor and mounted on the printed circuit board; and a metal base plate located at a bottom of the centrifugal fan, the metal base plate being in contact with a member and the member being in contact with the electronic component.
 7. The centrifugal fan according to claim 6, wherein the stationary assembly is provided with a radiation fin attached to the base plate for radiating heat.
 8. The centrifugal fan according to claim 2, wherein the stationary assembly further includes: a printed circuit board attached on the stationary assembly; an electronic component having a driving circuit for the three phase motor and mounted on the printed circuit board; and a metal part made of a metal formed in at least a part of the housing, wherein the electronic component is in contact with at least a part of the metal part of the housing.
 9. The centrifugal fan according to claim 2, wherein the stationary assembly further includes: a printed circuit board attached on the stationary assembly; an electronic component having a driving circuit for the three phase motor and mounted on the printed circuit board; and a metal part made of a metal formed in at least a part of the housing, wherein the electronic component is in contact with a member and the member is in contact with at least a part of the metal part of the housing.
 10. The centrifugal fan according to claim 2, wherein the stationary assembly further includes: a printed circuit board attached on the stationary assembly; an electronic component having a driving circuit for the three phase motor and mounted on the printed circuit board; and a metal part made of a metal formed at a part of the housing, wherein the metal part is disposed in an area in which the plural vanes are arranged in the housing in the axial direction and the electronic component is in contact with the outer surface of the metal part.
 11. The centrifugal fan according to claim 2, wherein the stationary assembly further includes: a printed circuit board attached on the stationary assembly; an electronic component having a driving circuit for the three phase motor and mounted on the printed circuit board; and a metal part made of a metal formed at a part of the housing, wherein the metal part is disposed in an area in which the plural vanes are arranged in the housing in the axial direction and the electronic component is in contact with a member and the member is in contact with at least a part of the metal part of the housing.
 12. The centrifugal fan according to claim 1, wherein the central axis of the three phase motor coincides with a central axis of the impeller and an outer peripheral diameter of the three phase motor is smaller than that of the impeller.
 13. A centrifugal fan comprising: an impeller having a cylindrical outline shape; a three phase motor connected to the impeller and rotating the impeller at 10000 or more revolutions per minute around a central axis thereof; and a housing accommodating the impeller therein, wherein the motor includes: a rotor assembly supporting the impeller and having a rotor magnet disposed around the central axis thereof; and a stationary assembly having a stator generating a rotational torque between the rotor magnet and the stator, wherein the impeller includes plural vanes, each vane has a connection end at a lower and an upper end, and is disposed at an equal interval each other as a manner that the connection end is fixed on an upper surface of the rotor assembly, that each vane is disposed parallel to the central axis having a predetermined distance from the central axis in radial direction, and that the upper end of each vane is fixed on a ring as an open end, and the outline shape of the impeller satisfies a relation of 2r≦25 mm, where r is a radius of the impeller, and wherein the housing includes: an intake port formed on the open end of the impeller, from which an air is absorbed into the impeller when the impeller rotates; and an air blowing-out port formed on the housing, from which the air is flowing out to an outside of the centrifugal fan when the impeller rotates.
 14. The centrifugal fan according to claim 13, wherein electric currents are supplied at all times in all stator coils wound around the stator as a three phase motor.
 15. The centrifugal fan according to claim 14, wherein the stationary assembly further includes: a printed circuit board attached on the stationary assembly; an electronic component having a driving circuit for the three phase motor and mounted on the printed circuit board; and a metal base plate with which the electronic component is in contact.
 16. The centrifugal fan according to claim 15, wherein the stationary assembly is provided with a radiation fin attached to the base plate for radiating heat.
 17. The centrifugal fan according to claim 14, wherein the stationary assembly further includes: a printed circuit board attached on the stationary assembly; an electronic component having a driving circuit for the three phase motor and mounted on the printed circuit board; and a metal base plate located at a bottom of the centrifugal fan, the metal base plate being in contact with a member and the member being in contact with the electronic component.
 18. The centrifugal fan according to claim 17, wherein the stationary assembly is provided with a radiation fin attached to the base plate for radiating heat.
 19. The centrifugal fan according to claim 13, wherein the rotor magnet is a rare earth bond magnet.
 20. The centrifugal fan according to claim 13, wherein the central axis of the three phase motor coincides with a central axis of the impeller and an outer peripheral diameter of the three phase motor is smaller than that of the impeller. 